Using these sequences, every person could be identified solely by the sequence of their base pairs. However, because there are so many millions of base pairs, the task would be very time-consuming. Instead, scientists are able to use a shorter method, because of repeating patterns in DNA.
These patterns do not, however, give an individual “fingerprint,” but they are able to determine whether two DNA samples are from the same person, related people, or non – related people. Scientists use a small number of sequences of DNA that are known to vary among individuals a great deal, and analyse those to get a certain probability of a match.
(Source 2)
How is it done?
Because fingerprints are unique to a person, so no one person can have the same fingerprint, we can use them to identify Criminals and solve homicide cases. Heres how it works:
- You take some DNA and cut it with restriction enzymes. These chemicals cut the long DNA molecule each time they recognise a certain sequence of sub units, or bases. The goal of this step is to cut up the DNA evidence into manageable fragments.
- Then you put the fragments on a layer of gelatine and hook up an electric field to the ends of the gelatine.
- When you switch on the current, it attracts the fragments across the field.
- The shorter fragments move faster than the long ones, and that sorts the fragments by size.
- You attach radioactive markers to certain sequences of the DNA. For example you might label every occurrence of DADDADCAT – a different series of “letters” in DNA’s alphabet.
- You place the markers against a photographic film, and produce a picture showing the individual markers.
- Now you repeat the process with different markers, to multiply the accuracy of the test.
(Source 4)
This process is known as Restriction Fragment Length Polymorphism. However a process called Polymerase chain reaction is used to produce multiple copies of segments from a very little amount of DNA. This enables a DNA fingerprint to be made from a single hair. Once a sample has been produced, the pattern of the alleles from a limited number of genes is compared with the pattern from the reference sample. A non match is conclusive, but the technique provides less certainty when a match occurs.
(Source 7)
However DNA Fingerprints aren’t just used to identify and convict criminals, they can be used to determine the paternity of a child. To do this there is a process Called VNTRs.
Every strand of DNA has pieces that contain genetic information which informs an organism’s development (exons) and pieces that, apparently, supply no relevant genetic information at all (introns). Although the introns may seem useless, it has been found that they contain repeated sequences of base pairs. These sequences, called variable Number Tandem Repeats (VNTRs), can contain anywhere from twenty to one hundred base pairs.
Every human being has some VNTRs. To determine if a person has a particular VNTR, a southern blot is performed, and then the southern blot is probed, through a hybridization reaction, with a radioactive version of the VNTR in question. The pattern which results from this process is what is often referred to as a DNA Fingerprint.
A given persons VNTRs come from the genetic information donated by his or her parents; he or she could have VNTRs inherited from his or her mother or father, or a combination, but never a VNTR either of his or her parents do not have.
Because VNTR patterns are inherited genetically, a given persons VNTR pattern is more or less unique. The more VNTR probes used to analyse a persons VNTR pattern, the more distinctive and individualized that pattern, or DNA fingerprint will be
(Source 6)
Southern Blot
- In order to analyse a southern blot, a radioactive genetic probe is used in a hybridisation reaction with the DNA in question. If an x ray is taken of the southern blot after a radioactive probe has been allowed to bond with the denatured DNA on the paper, only the areas where the radioactive probe binds will show up on the film. This allows researchers to identify, in a particular persons DNA, the occurrence and frequency of the particular genetic pattern contained in the probe.
(Source 5)
Practical applications of DNA Fingerprinting.
As I said before DNA can be used to determine some bodies paternity, through the use of VNTR pattern analysis which has been known to solve standard father – identification cases as well as more complicated cases of confirming legal nationality and, in instances of adoption, biological parenthood.
DNA fingerprinting can be used in criminal identification and forensics, as well as personal identification. The notion of using DNA fingerprints as a sort of genetic bar code to identify individuals has been discussed, but this is not likely to happen anytime soon or in the foreseeable future! The technology required to isolate, keep on file, and then analyse millions of very specified VNTR patterns is both highly expensive and impractical – not to mention degrading for people! However it would come in handy when we need to solve a murder!
(Sources 9, 8, 10 & 11)